ABSORPTION AND FUNCTION OF ZINC IN PLANTS
Zinc is taken up predominantly as a divalent cation (Zn2   ), but at high pH it is probably absorbed as a monovalent cation (ZnOH  ) . Zinc is either bound to organic acids during long distance transport in the xylem or may move as free divalent cations. Zinc concentrations are fairly high in phloem sap where it is probably complexed to low-molecular-weight organic solutes . The metabolic functions of zinc are based on its strong tendency to form tetrahedral complexes with N-, O-, and particularly S-ligands, and thus it plays a catalytic and structural role in enzyme reactions .
 Zinc is an integral component of enzyme structures and has the following three functions: catalytic, coactive, or structural . The zinc atom is coordinated to four ligands in enzymes with catalytic functions. Three of them are amino acids, with histidine being the most frequent, followed by glutamine and asparagine. A water molecule is the fourth ligand at all catalytical sites. The structural zinc atoms are coordinated to the S-groups of four cysteine residues forming a tertiary structure of high stability. These structural enzymes include alcohol dehydrogenase, and the proteins involved in DNA replication and gene expression . Alcohol dehyrogenase contains two zinc atoms per molecule, one with catalytic reduction of acetaldehyde to ethanol and the other with structural functions. Ethanol formation primarily occurs in meristematic tissues under aerobic conditions in higher plants. Alcohol dehyrdrogenase activity decreases in zinc-deficient plants, but the consequences are not known . Flooding stimulates the alcohol dehydrogenase twice as much in zinc-sufficient compared with zinc-deficient plants, which could reduce functions in submerged rice .
 Carbonic anhydrase (CA) contains one zinc atom, which catalyzes the hydration of carbon dioxide (CO2). The enzyme is located in the chloroplasts and the cytoplasm. Carbon dioxide is the substrate for photosynthesis in C3  plants, but no direct relationship was reported between CA activity and photosynthetic CO2 assimilation in C3 plants . The CA activity is absent when zinc is extremely low, but when even a small amount of zinc is present, maximum net photosynthesis can occur. Photosynthesis by C4  metabolism is considerably different  than that occurring in C3  plants. For C4  metabolism, a high CA activity is necessary to shift the equilibrium in favor of HCO3   for phosphoenolpyruvate carboxylase, which forms malate for the shuttle into the bundle sheath chloroplasts, where CO2 is released and serves as substrate of ribulosebisphosphate carboxylase.

3 ZINC DEFICIENCY
Zinc deficiency is common in plants growing in highly weathered acid or calcareous soils . Roots of zinc-deficient trees often exude a gummy material. Major zinc-deficient sites are old barnyards or corral sites, where an extra heavy manure application accumulated over the years. Zinc ions become tied to organic matter to the extent that zinc is not available to the roots of peach trees . Zinc deficiency initially appears in all plants as intervenial chlorosis (mottling) in which lighter green to pale yellow color appears between the midrib and secondary veins Developing leaves are smaller than normal, and the internodes are short. Popular names describe these conditions as ‘little leaf’ and ‘rosette’ . Pecan trees in particular suffer

ZINC TOLERANCE
Zinc is the heavy metal most often in the highest concentrations in wastes arising in industrialized communities . Zinc exclusion from uptake, or binding in the cell walls, does not seem to contribute to zinc tolerance . Zinc exclusion might exist in Scots pine (Pinus sylvestris L.), where certain ectomycorrhizal fungi retain most of the zinc in their mycelia, resulting in the ability of the plant to tolerate zinc . Infections with ectomycorrhizal fungi are beneficial for the growth and development of pecan . These fungi are highly specialized parasites that do not cause root disease. They are symbiotic, thus gaining substance from the root and contributing to the health of the root.
 Tolerance is achieved through sequestering zinc in the vacuoles, and zinc remains low in the cytoplasm of tolerant plants, whereas zinc is stored in the cytoplasm of non-tolerant clones . Positive correlation between organic acids such as citrate and malate with zinc in tolerant plants indicates a mechanism of zinc tolerance . Zinc tolerance in tufted hair grass (Deschampsia caespitosa Beauvois) was increased in plants supplied with ammonium as compared to nitrate nutrition. This effect apparently is caused by greater accumulation of asparagine in the cytoplasm of ammonium-fed plants, which form stable complexes with asparagines and zinc .

PHOSPHORUS-ZINC INTERACTIONS
The higher phosphorus content in zinc-deficient plants supplied with high phosphorus can to some degree be attributed to a concentration effect . However, the main reason for the high concentration in the leaves is that zinc deficiency enhances the uptake rate of phosphorus by the roots and translocation to the shoots . This enhancement effect is specific for zinc deficiency and is not observed when other micronutrients are deficient. Enhanced phosphorus uptake in zinc-deficient plants can be part of an expression of higher passive permeability of the plasma membranes of root cells or impaired control of xylem loading. Zinc-deficient plants also have a high phosphorus content because the retranslocation of phosphorus is impaired.

 TRYPTOPHAN AND INDOLE ACETIC ACID SYNTHESIS
The most distinct zinc deficiency symptoms are ‘little leaf’ and ‘rosette’ in pecans and peaches . These symptoms have long been considered to represent problems in indole acetic acid (IAA, auxin) metabolism. However, the mode of action of zinc in auxin metabolism is unidentified. Retarded stem elongation in zinc-deficient tomato (Lycopersicon esculentum Mill.) plants was correlated with a decrease in IAA level, but resumption of stem elongation and IAA content occur after zinc is resupplied. Increased IAA levels preceded elongation growth upon resupply of zinc , which would be expected if growth was a response of increased supply of auxin caused by application of zinc. Low levels of IAA in zinc-deficient plants are probably the results of inhibited synthesis of IAA . There is an increase in tryptophan content in the dry matter of rice (Oryza sativa L.) grains by zinc fertilization of plants grown in calcareous soil . The lower IAA content in zinc-deficient leaves may be due to the biosynthesis of IAA tryptophan . Lower IAA contents may be the result of enhanced oxidative degradation of IAA caused by superoxide generation enhanced under conditions of zinc deficiency .